In this study, flood frequency, productivity, and spatial heterogeneit
y were correlated with plant species richness (SX) among wetlands on a
coastal island in southeast Alaska. Studies of 16 sites in or near th
e Kadashan River basin demonstrated nonlinear, unimodal relations betw
een flood frequency and SR, productivity and SR, and linear relations
between SR and the spatial variation of flood frequencies (SVFF) withi
n a site. SVFF is caused by microtopographic variation in elevation. A
nonlinear regression model relating SR to flood frequency and SVFF ex
plained much of the variation in SR between wetland communities. Sites
with intermediate flood frequencies and high SVFF were species-rich,
while sites frequently, rarely, or permanently flooded and with low SV
FF were species-poor. The data suggest that small-scale spatial variat
ion can dramatically alter the impact of disturbances. The data also s
upport Michael Huston's dynamic-equilibrium model of species diversity
, which predicts the effects of productivity and disturbance on divers
ity patterns. Species-rich sites had low to intermediate levels of pro
ductivity and intermediate flood frequencies, and species-poor sites h
ad very low or high flood frequencies and low productivity, supporting
the model's predictions. The model was tested at contrasting spatial
scales (1000 m(2) and 1 m(2)). At the 1000-m(2)? scale, Huston's model
predicted 78% of the variation in SR. At the microplot scale, relatio
nships between SR and flood frequency were weaker, and the dynamic-equ
ilibrium model predicted only 36%; of the variation in SR.